1. Field of the Invention
This invention relates to a process for preparing dimethylnaphthalene using a zeolite catalyst. Specifically it relates to a process for alkylating toluene and dehydrocyclizing the alkylate to produce a product rich in 2,6-dimethylnaphthalene.
2. Background of the Art
The compound 2,6-dimethyl naphthalene is a high valued commodity chemical that is used in the manufacture of polymers and polyester resins. Up to now, some of the methods for producing dimethylnaphthalene have required expensive starting materials such as 4-pentenyl-toluene; 5-(o-tolyl)-pent-2-ene; and 1,3-dimethyl-4-isobutylbenzene. These materials are cyclized, usually by means of a noble metal/alumina catalyst.
Dimethylnaphthalenes can also be produced by alkylating naphthalene and/or methylnaphthalene with methylating agent(s) in the presence of crystalline silicates with a moles ratio of SiO.sub.2 /M.sub.2 O.sub.3 of 12 or more, wherein M is typically aluminum.
Isomerization using zeolite catalysts has also been employed.
For example, U.S. Pat. No. 4,777,312 (Bakas et al.) describes a method for isomerization of a feedstock of mixed dimethylnaphthalenes to produce a higher concentration of 2,6-dimethylnaphthalene isomer than in the feedstock. This method employs mordenite type zeolite as a catalyst.
U.S. Pat. No. 3,775,496 (Thompson) describes a multistep process for the preparation of 2,6-dimethylnaphthalene and 2,7-dimethylnaphthalene from 5-0-tolyl-pentene-2. The 5-0-tolyl-pentene-2 is first converted to a mixture of 1,6- and 1,8-dimethyltetralins, which are then dehydrogenated to the respective dimethylnaphthalenes. The dimethylnaphthalenes (DMN) are then isomerized respectively to 2,6-dimethylnaphthalene and 2,7-dimethylnaphthalene.
U.S. Pat. No. 4,795,847 (Weitkamp et al.) discloses a process for the preparation of 2,6-dialkylnaphthalenes by selectively alkylating naphthalene or 2-alkyl-naphthalene with an alkylating agent such as methanol in the presence of a zeolite ZSM-5 catalyst.
United Kingdom Patent No. GB 2,133,032 discloses a method for preparing dimethylnaphthalenes by reacting monomethyl naphthalene with methanol in the presence of an alumina, silica, or silica-alumina catalyst.
German Patent No. DE 3,334,084 mentions the use of ZSM-5 as a catalyst for the alkylation of naphthalene or alkylnaphthalene with methanol or dimethyl ether.
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties. Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline silicates. These silicates can be described as a rigid three-dimensional framework of SiO.sub.4 and Periodic Table Group IIIA element oxide, e.g., AIO.sub.4, in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total Group IIIA element, e.g., aluminum, and silicon atoms to oxygen atoms is 1:2 . The electrovalence of the tetrahedra containing the Group IIIA element, e.g., aluminum, is balanced by the inclusion in the crystal of a cation, e.g., an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of the Group IIA element, e.g., aluminum, to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given silicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Alkylation is one of the most important and useful reactions of hydrocarbons. Lewis and Bronsted acids, including a variety of natural and synthetic zeolites, have been used as catalysts. Alkylation of aromatic hydrocarbon compounds employing certain crystalline zeolite catalysts is known in the art. For instance, U.S. Pat. No. 3,251,897 describes liquid phase alkylation in the presence of crystalline aluminosilicates such as faujasite, heulandite, clinoptilolite, mordenite, dachiardite, zeolite X and zeolite Y. The temperature of such alkylation procedure does not exceed 600.degree. F., thereby maintaining patentee's preferable operating phase as substantially liquid.
Also, U.S. Pat. No. 2,904,607 shows alkylation of hydrocarbon compounds in the presence of certain crystalline zeolites. The zeolites described for use in this patent are crystalline metallic aluminosilicates, such as, for example, magnesium aluminosilicate.
U.S. Pat. Nos. 3,631,120 and 3,641,177 describe liquid phase processes for alkylation of aromatic hydrocarbons with olefins in the presence of certain zeolites. U.S. Pat. No. 3,631,120 discloses use of an ammonium exchanged, calcined zeolite having a silica to alumina mole ratio of between 4.0 and 4.9. U.S. Pat. No. 3,641,177 discloses use of a zeolite catalyst activated in a particular manner.
U.S. Pat. Nos. 3,751,504 and 3,751,506 describe the vapor phase alkylation of aromatic hydrocarbons with olefins in the presence of a specified type of zeolite catalyst.
U.S. Pat. Nos. 3,755,483 and 4,393,262 disclose the vapor phase reaction of propylene with benzene in the presence of zeolite ZSM-12, to produce isopropylbenzene.
U.S. Pat. No. 4,469,908 discloses the alkylation of aromatic hydrocarbons with relatively short chain alkylating agents having from one to five carbon atoms employing ZSM-12 as alkylation catalyst.
Harper et al. have described a catalytic alkylation of benzene with propylene over a crystalline zeolite (Petrochemical Preprints, American Chemical Society, Vol. 22, No. 3, p. 1084, 1977). Extensive kinetic and catalyst aging studies were conducted with a rare earth exchanged Y-type zeolite (REY) catalyst.